Method

• 1.
  Preheat the oven to 180°C. Arrange an oven rack in the centre position. Line a 20cm square cake pan with 2 sheets of foil, crossing one over the other and pressing into the corners and up the sides. Lightly butter the foil and set aside.
• 2.
  In a large heatproof bowl, whisk the cocoa powder and 1/4 cup (59g) boiling water until smooth (this will bring out the flavour of the cocoa). Add the semisweet chocolate, butter, and oil to the cocoa mixture and set over a medium saucepan filled with about 3cm of simmering (not boiling) water (make sure the bottom of the bowl isn’t touching the water). Warm the mixture gently, stirring occasionally, until chocolate and butter are melted and the mixture is smooth. Remove from the heat and let cool until lukewarm.
• 3.
  Whisk sugars into chocolate mixture. It will look grainy and you might see some of the fat start to separate, which is normal. Add whole egg, yolks, and vanilla and whisk vigorously until mixture comes back together and looks very thick, smooth, and glossy. Add flour, malted milk powder (if using), and salt and whisk slowly until combined, then whisk more vigorously until batter is very thick, 45 seconds.
• 4.
  Add the milk chocolate to the batter and fold with a flexible spatula to distribute. Scrape into prepared pan, spreading in an even layer all the way to the corners. Bake brownies until the surface is shiny and puffed and the centre is dry to the touch but still soft when pressed, 25-30 minutes.
• 5.
  Cool in the pan until brownies are no longer hot, about 1 hour, then refrigerate until the bottom of the pan feels cold, about 1 hour longer (this results in a chewy texture). Use the foil to lift brownies out of the pan and transfer to a cutting board. Slice the brownies into 16 squares.